EU notwithstanding, we don't know the circumstances of Plagueis' death. And Palpatine could have been lying his ass off to Anakin anyway. It's a little convenient to dangle the carrot of learning how cheat death in front of him just as he's starting to have visions of Padme dying.
Going by the lightsaber, I think it's someone much older, who has just woken up, and/or someone who was too far away from the action in the previous films to be detected using the Force.